Apple’s OS X is vulnerable to the Shellshock bug, but it’s not that easy for attackers to take advantage of it, according to Intego, which specializes in security software for the operating system.

Shellshock is the nickname for a flaw in the Bourne Again Shell, or Bash, which is a command-line shell processor widely present in Unix and Linux systems. The flaw in Bash, which has been present for two decades, could allow an attacker to take complete control of a computer.

Apple, which plans to patch the flaw, said most users are fine unless they’ve tweaked advanced Unix settings. By adjusting those settings, Bash could be exposed to attackers, wrote Derek Erwin of Intego in a blog post. Intego has already seen proof-of-concept exploits for OS X, he wrote.

For example, Bash would be exposed if a user turned on the remote login capability for all users, including guests. But that is an action that “is probably not the most secure thing to do anyway,” Erwin wrote, as it would open up the computer to other possible attacks.

Another scenario in which adjusted settings could make a difference is on a Lion OS X server running Apache or PHP scripting environments, Erwin wrote. If Apache is configured to run scripts, an attacker could insert variables into a script that a Bash shell would run.

“This one, however, requires exploiting two holes,” he wrote. “First, in the script running on Apache, and then in turn using that compromised script to send something to the Bash shell.”

Both scenarios, however, are “edge cases,” he wrote, “and both routes probably require a level of technical expertise that the person configuring their account as such can patch the exploit fairly simply.”

Computer security researchers are still trying to figure out the extent to which Internet-connected devices are vulnerable to Bash, as the software is so prevalent. Attackers have been scanning the Internet to find vulnerable systems, and experts are predicting that more specific attacks customized for vulnerable systems will emerge.

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